diff --git a/example/imageviewer/imageviewer.cc b/example/imageviewer/imageviewer.cc
index 2dd109b..73a07c1 100644
--- a/example/imageviewer/imageviewer.cc
+++ b/example/imageviewer/imageviewer.cc
@@ -111,7 +111,7 @@
   }
 
   DecodeImageCallbacks::AllocResult  //
-  OnImageConfig(const wuffs_base__image_config& image_config) {
+  OnImageConfig(const wuffs_base__image_config& image_config) override {
     uint32_t w = image_config.pixcfg.width();
     uint32_t h = image_config.pixcfg.height();
     if ((w == 0) || (h == 0)) {